Method of application of conductive cap-layer in flip-chip, COB, and micro metal bonding

ABSTRACT

A method of bonding a bonding element to a metal bonding pad comprises the following steps. A semiconductor structure having an exposed, recessed metal bonding pad within a layer opening is provided. The layer has an upper surface. A conductive cap having a predetermined thickness is formed over the metal bonding pad. A bonding element is bonded to the conductive cap to form an electrical connection with the metal bonding pad.

BACKGROUND OF THE INVENTION

Copper (Cu) is gradually replacing aluminum (Al) as the interconnectmaterial in integrated circuits. However, a problem arises due to thefact that the top layer Cu pad does not form a good connection with thenormal bonding techniques. This is mainly due to the poor quality ofcopper oxide formed on the Cu pad surface upon exposure to theatmosphere and moisture.

In order to ensure a good contact between the chip and the bondingelement, either the top Cu metal layer has to be replaced by Al, or anindirect way of bonding is needed. Both of these options increase thecomplexity and cost.

U.S. Pat. No. 5,384,284 to Doan et al. describes a process to form a padinterconnect whereby a metal layer or a metal alloy is bonded to anunderlying aluminum pad by chemical vapor deposition (CVD) or byelectroless deposition. A conductive epoxy film is then adhered to themetal layer. The metal layer may be comprised of, for example, Cu, Ni,W, Au, Ag, or Pt and the metal alloy may be comprised of titaniumnitride, for example.

U.S. Pat. No. 5,462,892 to Gabriel describes a method of processing asemiconductor wafer so as to inhibit corrosion of aluminum or othermetal interconnection lines. Such interconnection lines may be composedof tungsten (W)/titanium (Ti), aluminum (Al), aluminum-copper (Cu), orsuccessive layers of W/Ti and Al—Cu. Once the interconnection lines areetched, the wafer is moved from an etching chamber a post-etchingprocessing chamber, without exposure to the atmosphere, where a thinnative oxide is formed on the exposed sidewalls of the just etchedinterconnection lines. The oxide layer protects the wafer surface fromacidic corrosion upon subsequent exposure to the atmosphere.

U.S. Pat. No. 5,985,765 to Hsiao et al. describes a method for reducingbonding pad loss using a capping layer, preferably comprised oftungsten, when contact openings are etched to the bonding pads whilemuch deeper fuse openings are concurrently etched.

U.S. Pat. No. 5,376,235 to Langley describes a process of using a wetchemical process to remove chlorine from dry etched metal features on asemiconductor wafer before an alloy step. The wet chemical processincludes a 20:1 phosphoric acid solution dip that prevents the formationof voids in the etched metal features during subsequent alloying stepsup to about 425° C. by removing chlorine.

U.S. Pat. No. 5,785,236 to Cheung et al. describes a process for formingelectrical connection between metal wires and metal interconnections nototherwise bondable, i.e. gold and aluminum wires and copperinterconnects. The copper pads are modified, by forming an aluminum padthereover, to permit the use of conventional wire bonding techniques.

U.S. Pat. No. 5,891,756 to Erickson describes a method for forming asolder bump pad, and specifically to converting a wire bond pad of asurface-mount IC device to a flip-chip solder bump pad such that the ICdevice can be flip-chip mounted to a substrate. The method uses a Nilayer over the pad.

U.S. Pat. No. 5,647,942 to Haji describes a wire bonding methodincluding the step of removing a thin surface layer of an electrodecomprising a copper layer and a nickel layer formed on the surface ofthe copper layer and coated with gold on the surface. This removesnickel hydroxide and nickel oxide present on the gold film.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provide amethod of bonding a metal wire to a copper bonding pad.

A further object of the present invention is to provide a method ofpreparing a copper bonding pad to permit the use of normal bondingtechniques to bond a wire to the copper bonding pad.

Another object of the present invention is to provide a method ofbonding a metal wire to a copper bonding pad with good adhesion.

Yet another object of the present invention is to provide a method ofbonding a metal wire to a copper bonding pad with minimum processcomplexity.

Other objects will appear hereinafter.

It has now been discovered that the above and other objects of thepresent invention may be accomplished in the following manner.Specifically, a semiconductor structure having an exposed, recessedmetal bonding pad within a layer opening is provided. The layer has anupper surface. A conductive cap having a predetermined thickness isformed over the metal bonding pad. A bonding element is bonded to theconductive cap to form an electrical connection with the metal bondingpad.

BRIEF DESCRIPTION OF THE DRAWINGS

The features and advantages of the method of bonding a wire to a metalbonding pad according to the present invention will be more clearlyunderstood from the following description taken in conjunction with theaccompanying drawings in which like reference numerals designate similaror corresponding elements, regions and portions and in which:

FIG. 1 schematically illustrates in cross-sectional representation thebeginning structure for the embodiments of the present inventionaccording to the present invention.

FIG. 2 schematically illustrates in cross-sectional representation afirst embodiment of forming a recessed metal bonding pad structureaccording to the present invention.

FIGS. 3A and 3B schematically illustrate in cross-sectionalrepresentation a second embodiment of forming a recessed metal bondingpad structure according to the present invention.

FIG. 4 schematically illustrates in cross-sectional representation aconductive cap layer formed over the recessed metal bonding padstructure of either FIG. 2, or 3B according to the present invention.

FIGS. 5A-5C schematically illustrate in cross-sectional representation afirst embodiment for patterning the conductive cap layer of FIG. 4 overthe metal bonding pad according to the present invention.

FIGS. 6A-6C schematically illustrate in cross-sectional representation asecond embodiment for patterning the conductive cap layer of FIG. 4 onlyover the metal bonding pad according to the present invention.

FIG. 7 schematically illustrates in cross-sectional representation athird embodiment for defining the conductive cap layer of FIG. 4 onlyover the metal bonding pad according to the present invention.

FIG. 8 schematically illustrates in cross-sectional representation thepreferred embodiment for bonding a bonding element to the definedconductive cap layer structure of either FIGS. 5C, 6C, or 7C accordingto the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Starting Copper BondingPad Structure

Accordingly FIG. 1 shows a schematic cross-sectional diagram of metalbonding pad 14 within opening 15 of intermetal dielectric (IMD) layer 12of an integrated circuit that includes semiconductor structure 10.

Semiconductor structure 10 is understood to possibly include asemiconductor wafer or substrate, active and passive devices formedwithin the wafer, conductive layers and dielectric layers (e.g.,inter-poly oxide (IPO), intermetal dielectric (IMD), etc.) formed overthe wafer surface. The term “semiconductor structure” is meant toinclude devices formed within a semiconductor wafer and the layersoverlying the wafer.

IMD layer 12 is formed over semiconductor structure 10. Bonding padopening 15 is etched within IMD layer 12. Metal bonding pad 14 is formedwithin bonding pad opening 15

Unless otherwise specified, all structures, layers, etc. may be formedor accomplished by conventional methods known in the prior art.

Metal bonding pad 14 may be comprised of copper or a copper alloy suchas AlCu, and is preferably copper, and contacts underlying circuitry(not shown) covered by IMD layer 12. For purposes of illustration, metalbonding pad 14 will be considered comprised of copper hereafter in allsubsequent embodiments.

Methods of Recessing the Copper Bonding Pad

There are two different illustrated embodiments to effectively recesscopper bonding pad 14 starting with the structure shown in FIG. 1. Thefirst such embodiment is illustrated in FIG. 2 (wet stripping, oretching). The second such embodiment is illustrated in FIGS. 3A and 3B(normal passivation mask and etch). These two alternate methods offorming a copper recess will be discussed in turn.

I. As shown in FIG. 2 (the first copper recess embodiment example), thecopper recess may be formed by wet stripping, or etching, copper bondingpad 14 of FIG. 1 using HNO₃, O2/NH₃, O₂/ammonium salt, HF/CH₃COOH, orother conventional chemistries at room temperature, but time of etchingand concentration will affect the amount of copper etched which isdetermined by the depth of the recess needed.

This creates opening 19 within IMD layer 12 over recessed copper bondingpad 14′.

Recessed copper bonding pad 14′ so formed is recessed from about 500 to10,000 Å beneath the upper surface 12′ of IMD layer 12, and morepreferably from about 1000 to 5000 Å.

II. FIGS. 3A and 3B illustrate the second copper recess embodimentexample. As shown in FIG. 3A, passivation layer 16 is formed over copperbonding pad 14 and IMD layer 12 of FIG. 1.

As shown in FIG. 3B, passivation layer 16 may be masked (not shown) andetched to form opening 21 exposing copper bonding pad 14 and leavingpassivation layer portions 16′ on either side of copper bonding pad 14.

For either the FIG. 2 embodiment or the FIGS. 3A and 3B embodiment offorming recessed copper bonding pad 14′, 14, respectively, recessedcopper bonding pad 14′, 14 is recessed 17 from about 1000 to 10,000 Å,and more preferably from about 2000 to 7000 Å beneath the surface 12′,16″ of patterned IMD layer 12 and patterned passivation layer 16′,respectively.

Methods of Forming a Conductive Cap Layer Over the Recessed CopperBonding Pad

In a key step of the invention, and in whichever embodiment is used toform recessed copper bonding pad 14′, 14, conductive cap layer,preferably comprised of titanium (Ti), tantalum (Ta), tungsten (W), orother metal or conductive compound which has a good adhesion between theCu bonding pad 14′, 14 and the bonding element 22, in all embodiments ofthe present invention, is then formed over recessed copper bonding pad14′ 14 as shown in FIGS. 4, 5A-5C, 6A-6C, and 7. For purposes ofsimplicity, patterned passivation layer 16′ will not be shown.

As shown in FIG. 4, conductive cap layer 18 is formed over recessedcopper bonding pad 14′, 14 and either patterned IMD layer 12 orpatterned passivation layer 16′ (not shown). Conductive cap 18 may beformed by a PVD or CVD method. Conductive cap layer 18 is from about 500to 5000 Å thick, and more preferably from about 1000 to 4000 Å.

There are three different illustrated embodiments to pattern conductivecap layer 18 so as to only cover recessed copper bonding pad 14′, 14.The first such embodiment is illustrated in FIGS. 5A-5C(spin-on-material deposition and blanket etch back). The second suchembodiment is illustrated in FIGS. 6A-6C (photoresist masking andetching). The third such embodiment is illustrated in FIG. 7 (chemicalmechanical polishing).

I. FIGS. 5A-5C illustrate the first conductive cap layer 18′ formationembodiment example (spin-on-material deposition and blanket etch back).As shown in FIG. 5A, spin-on-coating 20 is deposited over conductive caplayer 18 of FIG. 4 preferably by a spin-on technique.

Spin-on-coating 20 may be comprised of photoresist, polyimide, organicplanarization BARC, or spin-on-glass (SOG) and is preferably comprisedof photoresist. Spin-on-coating 20 is generally thicker over opening 15and thinner over the surface 12′ of patterned IMD layer 12 due toplanarization effect.

As shown in FIG. 5B, a blanket etch is then performed, removing thatportion of spin-on-coating 20 and conductive cap layer 18 over thesurface 12′ of patterned IMD layer 12, leaving some of spin-on-coating20 and all of conductive cap layer 18′ within opening 15 over recessedcopper bonding pad 14′, 14.

As shown in FIG. 5C, an ashing, a wet ashing, or the combination of thetwo, and a cleaning step removes the remaining portion ofspin-on-coating 20 within opening 15, leaving conductive cap layer 18′over recessed copper bonding pad 14′, 14.

II. FIGS. 6A-6C illustrate the second conductive cap layer 18′ formationembodiment example (photoresist masking and etching). As shown in FIG.6A, photoresist mask layer 21 is selectively formed over the portion ofconductive cap layer 18 within opening 15 of FIG. 4, by a reverse maskprocess or a reverse-tone resist process to mask that portion ofconductive cap layer 18 within opening 15.

As shown if FIG. 6B, the unmasked portion of conductive cap layer 18 isetched, preferably by a plasma etch, and removed.

As shown in FIG. 6C, photoresist mask 21 is removed, preferably byashing, and the structure is cleaned.

III. FIG. 7 illustrates the third conductive cap layer 18′ formationembodiment example (chemical mechanical polishing). As shown in FIG. 7,the conductive cap layer 18 of FIG. 4 is polished to remove the excessof conductive cap layer 18 over patterned IMD layer 12 or passivationlayer 16′ (not shown), preferably by chemical mechanically polishing(CMP).

Regardless of which of the three conductive cap layer 18′ formationembodiment examples are employed the resulting structures shown in FIGS.5C, 6C and 7 are essentially identical.

As shown in FIG. 8, bonding element 22 is positioned to contact, orconnect, to conductive cap layer 18′ at 24 of any of the structuresillustrated in FIGS. 5C, 6C, or 7. Conductive cap layer 18′ effectivelyacts as a good glue layer to permit bonding or attachment of bondingelement 22, allowing an electrical connection between bonding element 22and recessed copper bonding pad 14′, 14.

Bonding element 22 may be comprised of gold or aluminum, and ispreferably gold, and may be a wire. For purposes of illustration,bonding element 22 will be considered to be a gold wire hereafter.

Gold wire (bonding element) 22 is placed vertically (at an angle ofabout 90°) above conductive cap layer 18′.

Thus, by implementing the present invention, there is a minimumcompromise in the resistivity of the top layer, i.e. conductive caplayer 18′, as Cu instead of Al can be used as the bonding pad, i.e.recessed copper bonding pad 14′, 14. In addition, conductive cap layer18′ is a good glue layer between recessed copper bonding pad 14′, 14 andgold bonding element 22.

While particular embodiments of the present invention have beenillustrated and described, it is not intended to limit the invention,except as defined by the following claims.

We claim:
 1. A method of bonding a bonding element to a metal bondingpad, comprising the steps of: providing a semiconductor structure havingan exposed, recessed metal bonding pad within a layer opening; saidlayer opening having an upper surface; forming a conductive cap having apredetermined thickness only over said metal bonding pad; and bonding abonding element to said conductive cap to form an electrical connectionwith said metal bonding pad.
 2. The method of claim 1, wherein saidmetal bonding pad is comprised of a material selected from the groupcomprising copper and a copper alloy; and said bonding element iscomprised of a material selected from the group comprising aluminum andgold.
 3. The method of claim 1, wherein said metal bonding pad iscomprised of copper, and said bonding element is comprised of gold. 4.The method of claim 1, wherein said bonding element is a wire.
 5. Themethod of claim 1, wherein said conductive cap is from about 500 to 5000Å thick.
 6. The method of claim 1, wherein said conductive cap is fromabout 1000 to 4000 Å thick.
 7. The method of claim 1, wherein saidbonding element is positioned at an angle of about 90° relative to saidbonding pad.
 8. The method of claim 1, wherein said metal bonding pad isrecessed from about 500 to 5000 Å below said top surface of said layer.9. The method of claim 1, wherein said metal bonding pad is recessedfrom about 1000 to 5000 Å below said top surface of said layer.
 10. Amethod of bonding a bonding element to a metal bonding pad, comprisingthe steps of: providing a semiconductor structure having an exposed,recessed metal bonding pad within a layer opening; said layer openinghaving an upper surface; said metal bonding pad is comprised of amaterial selected from the group comprising copper and a copper alloy;forming a conductive cap having a predetermined thickness only over saidmetal bonding pad; and bonding a bonding element to said conductive capto form an electrical connection with said metal bonding pad; saidbonding element is comprised of a material selected from the groupcomprising aluminum and gold.
 11. The method of claim 10, wherein saidmetal bonding pad is comprised of copper, and said bonding element iscomprised of gold.
 12. The method of claim 10, wherein said bondingelement is a wire.
 13. The method of claim 10, wherein said conductivecap is from about 500 to 5000 Å thick.
 14. The method of claim 10,wherein said conductive cap is from about 1000 to 4000 Å thick.
 15. Themethod of claim 10, wherein said bonding element is positioned at anangle of about 90° relative to said bonding pad.
 16. The method of claim10, wherein said metal bonding pad is recessed from about 500 to 5000 Åbelow said top surface of said layer.
 17. The method of claim 10,wherein said metal bonding pad is recessed from about 1000 to 5000 Åbelow said top surface of said layer.